Traditional spectroscopic methods are limited in sensitivity to approximately one part per ten thousand (1:10.sup.4) to one part per hundred thousand (1:10.sup.5). The sensitivity limitation arises from instabilities in light source intensity that are translated into noise in the absorption signal. For general information on traditional spectroscopy methods see for example Dereniak and Crowe, Optical Radiation Detectors, John Wiley & Sons, New York, 1984, and Demtroder, Laser Spectroscoptpy, Springer, Berlin, 1996.
Cavity Ring-Down Spectroscopy (CRDS), a technique first described by O'Keefe and Deacon in an article in Rev. Sci. Instrum. 59(12):2544-2551 (1988), allows making absorption measurements with sensitivities on the order of one part per ten million (1:10.sup.7) to one part per billion (1:10.sup.9) or higher. For general information on CRDS see U.S. Pat. No. 5,528,040 by Lehmann, herein incorporated by reference, as well as the articles by Romanini and Lehmann in J. Chem. Phys. 102(2):633-642 (1995), Meijer et al. in Chem. Phys. Lett. 217(1-2):112-116 (1994), Zalicki et al. in App. Phys. Lett. 67(1):144-146 (1995), Jongma et al. in Rev. Sci. Instrum. 66(4):2821-2828 (1995), and Zalicki and Zare in J. Chem. Phys. 102(7):2708-2717 (1995).
In a conventional CRDS system, the sample (absorbing material) is placed in a high-finesse stable optical resonator consisting of two spherical mirrors facing each other along a common optical axis. Light incident on one mirror circulates back and forth multiple times in the resonator, setting up standing waves having periodic spatial variations. Light exiting through the other mirror measures the intracavity light intensity.
The radiant energy stored in the resonator decreases in time (rings-down). For an empty cavity, the stored energy follows an exponential decay characterized by a ring-down rate that depends only on the reflectivity of the mirrors, the separation between the mirrors, and the speed of light in the cavity. If a sample is placed in the resonator, the ring-down is accelerated; under suitable conditions, the intracavity energy decays almost perfectly exponentially. An absorption spectrum for the sample is obtained by plotting the reciprocal of the ring-down rate versus the wavelength of the incident light. CRDS has been applied to numerous systems in the visible, ultraviolet, and infrared. For information on the use of CRDS for spectroscopy in the visible, see the articles by Engeln and Meijer in Rev. Sci. Instrum. 67(8):2708-2713 (1996), Martin et al. in Chem. Phys. Lett. 258(1-2):63-70 (1996), Paul et al. in J. Chem. Phys. 104(8):2782-2788 (1996), Scherer et al. in J. Chem. Phys. 103(21):9187-9192 (1995), Scherer et al. in J. Chem. Phys. 102(13):5190-5199 (1995), Scherer et al. in Chem. Phys. Lett. 242(4-5):395-400 (1995). Heustis et al. in Canadian J. Phys. 72(11-12):1109-1121 (1994), and O'Keefe et al. in Chem. Phys. Lett. 172(3-4):214-218 (1990). Information on CRDS applications in the ultraviolet can be found in the above-referenced articles by Romanini and Lehmann, and Zalicki et al., as well as the articles by Zhu et al. in Chem. Phys. Lett. 257(5-6):487-491 (1996), Romanini and Lehmann in J. Chem. Phys. 105(1):81-88 (1996), Romanini and Lehmann in J. Chem. Phys. 105(1):68-80 (1996), Wahl et al. in Diamond and Related Materials 5(3-5):373-377 (1996) Boogaarts and Meijer in J. Chem. Phys. 103(13):5269-5274 (1995) , Zalicki et al. in Chem. Phys. Lett. 234(4-6), 269-274 (1995), Jongma et al. in J. Molecular Spectroscopy 165(2):303-314 (1994), and Romanini and Lehmann in J. Chem. Phys. 99(9):6283-6301 (1993). For information on the use of CRDS for infrared spectroscopy see the above-referenced article by Martin et al. (1996), as well as the article by Scherer et al. in Chem. Phys. Lett. 245(2-3):273-280 (1995).
In comparison to conventional spectroscopy techniques, CRDS is advantageous because of the increased pathlength due to multiple reflections. CRDS is also advantageous because of its relative insensitivity to variations in the amplitude of light generated by the light source. In a CRDS system, fluctuations in the intensity of the light source do not typically limit sensitivity.
Much of current ring-down spectroscopy still relies on fairly costly laser sources. As solid state lasers (e.g. Ti:Sapphire lasers, Nd:Yag-pumped OPOs, and ECDLs) have gained in reliability, tuning range, and output power, they have started to replace the more traditional tunable dye lasers, although they are no less expensive. Simultaneously, semiconductor diode lasers (DL) have also been improving in power, wavelength coverage, and reliability. The rapid growth of the communications industry in recent years has resulted in the availability of tunable near-infrared DLs at a rapidly diminishing cost. In fact, owing to their compactness, low cost, durability, high wallplug efficiency, and compatibility with both fiber and silicon technologies, infrared diode lasers are a promising light source for practical CRDS systems.
Early attempts have demonstrated difficulties in using DL sources in conventional CRDS systems. As long as the linewidth of the resonator is much narrower than the laser linewidth (as in most systems), only a small fraction of the light incident on the resonator enters the resonator; most of the light is reflected back toward the laser. Even under optical isolation, the optical feedback typically results in excess noise in the laser's frequency stability, mode oscillation stability, and power output. The excess laser noise leads to unstable laser-resonator coupling, increased baseline noise, and reduced absolute sensitivity for absorption measurements. Back-reflections also lead to an increase in laser linewidth, which limits spectral resolution. At high feedback levels typical in most practical setups, a wide variety of effects ranging from linewidth broadening to complete `coherence collapse` (linewidth &gt;10 GHz) are often observed (see FIG. 2).
The DL instabilities are due to `external cavities` formed between reflective optics and the facets of the DL. Since diode lasers typically have large gain bandwidths (around 40 nm), virtually any external cavity can become optically coupled to the diode laser and influence its behavior. Because the output facet of the DL has a low reflectivity or is antireflection (AR)-coated to increase output power, the dominant laser cavity (the cavity determining laser behavior) in a conventional system is formed between the back facet of the DL and the resonator input, rather than between the DL facets. The dominant external cavity affects both the gain and phase relations of the DL, as explained for example by Petermann in Laser Diode Modulation and Noise, Kluwer Academic Publishers, Dordrecht, 1988, p. 250-285, Agrawal and Dutta in Semiconductor Lasers, Van Nostrand Reinhold, New York, 1993, p. 258-309, and Coldren and Corzine in Diode Lasers and Photonic Integrated Circuits, John Wiley and Sons, New York, 1995, p. 221-257. Whenever back reflection is allowed, the lasing characteristics of the DL (frequency, number of excited modes, output power, etc.) typically become highly dependent on uncontrollable experimental parameters, most notably external cavity length(s).
A CRDS system making use of a diode laser as a light source is described in the article by Romanini et al. in Chem. Phys. Lett. 270:538-545 (1997), herein incorporated by reference. Romanini et al. noted that without stringent optical isolation (in excess of 70 dB), any backreflections from the ring-down resonator caused serious mode-hopping and transverse mode excitation in their laser. Romanini et al. also noted that a single-pass high-dB Faraday isolator was "absolutely needed," and that an acousto-optic modulator alone was not adequate for optical isolation as in the case of dye lasers.
The above-incorporated U.S. Pat. No. 5,528,040 (Lehmann) suggests the use of a diode laser in a continuous-wave (c.w.) CRDS system. The system described by Lehman did not ensure adequate optical isolation and was thus subject to significant diode laser instability problems. Even the substantial isolation level (60 dB) used by Lehmann was not sufficient for adequately reducing laser instability caused by back reflections into the diode laser.
A preliminary report on the use of c.w. diode lasers for CRDS can be found in the abstract by Romanini et al. in Proceedings of the 50th International Symposium on Molecular Spectroscopy, ed. T. A. Miller (Department of Chemistry, Ohio State University. Columbus, Ohio), 1995, p. 284. The potential use of c.w. diode lasers for CRDS is also discussed in the article by Romanini et al. in Chem. Phys. Lett. 264:316-322 (1997), herein incorporated by reference.
A solution to the coupling/feedback problem is described in the above-incorporated co-pending U.S. patent application Ser. No. 08/879,975. The feedback from the resonant cavity is completely eliminated through the use of a ring resonator geometry, in which reflections from the resonator input are not directed back towards the laser.
The present invention provides a novel solution to the problem of optical feedback from a resonant spectroscopy cavity to a diode laser. Optical feedback from a resonant cavity to a diode laser is of concern for various non-CRDS spectroscopy applications. For information on non-CRDS spectroscopy systems see for example U.S. Pat. Nos. 5,173,749 and 5,432,610. A method of stabilizing diode lasers in the presence of optical feedback from a resonant cavity would find use in many non-CRDS spectroscopy applications.